In a conventional pulse-width modulated inverter, the center-tapping capacitors C.sub.n are dimensioned for a sufficiently large capacitance that permits only an insignificant voltage ripple of the center point A of the tapping circuit at the operating frequency of the inverter. The main transformer is dimensioned for maximum inductance of L.sub.p, whereby its magnetization current remains small.
On the other hand, a conventional resonant inverter has the values of C and L.sub.p dimensioned so that the circuit sustains a large-amplitude resonance oscillation, whereby the peak-to-peak voltage at the center-tapping point A exceeds the voltage of the DC feed circuit.
Circuit configurations related to welding inverter techniques are described in, i.a., the following publications:
The U.S. Pat. No. 4,533,986 concerns a power supply for signal processing applications. The power supplies described are of the series resonant type.
The U.S. Pat. No. 4,679,129 describes an inverter configuration which uses series resonant power supplies.
The SE patent publication 386,330 describes an AC power source particularly for induction heating. The operating frequency of the power generator is determined by an external oscillator. The capacitors C5 and C6 of the circuit are dimensioned for resonance close to the operating frequency with the coil 7 acting as the load. The frequency of the oscillator 2 is adjustable to control the input power to the coil 7. The LC circuit is dimensioned to be approximately resonant at the operating frequency.
A crucial drawback of prior-art pulse-width modulated circuits has been associated with the cut-off behaviour of the switching elements at high current levels. If a transistor is used as the switching element, the matter is relatively insignificant at low current levels. However, the short-circuit currents occurring in welding at levels up to twice the nominal current pose problems during the cut-off phase in all inverter types.